1. Field of the Invention
The present invention relates to a display. More specifically, the present invention relates to a display for using Pulse Width Modulation (PWM) to represent brightness and gray scales.
2. Description of the Related Art
In general, a flat panel display (FPD) is a display device in which walls are provided between two substrates to form an airtight device, and appropriate elements are arranged in the airtight device to display desired images. The importance of the FPD has been emphasized following the development of multimedia technologies. In response to this trend, various flat displays such as a liquid crystal display (LCD), a plasma display panel (PDP), and an electron emission device (EED) have been put to practical use.
In particular, since an electron emission device uses phosphorous emission caused by electron beams in a like manner of the cathode ray tube (CRT), it has a high probability of realizing a flat-type display which maintains excellent features of the CRT, provides no image distortion, and allows low power consumption. In particular, it satisfies view angle, high-rate response, high resolution, fineness, and slimness criteria, and accordingly, it has become the center of public attention as a next-generation display.
Generally, there are two kinds of EED. One uses a thermionic (or hot) cathode as an electron source and the other uses a cold cathode as an electron source. Also, in EEDs using a cold cathode, there are field emitter array (FEA) type EEDs, surface conduction emitter (SCE) type EEDs, metal-insulator-metal (MIM) or metal-insulator-semiconductor (MIS) type EEDs, and ballistic electron surface emitting (BSE) type EEDs.
A typical EED is composed of a triode structure having cathode, anode, and gate electrodes. More specifically, the cathode electrode (generally used as a data electrode) is formed on a substrate. An insulation layer has a contact hole. The contact hole and the gate electrode (generally used as a scan electrode) are integrated on the insulation layer. Additionally, an emitter used as an electron source is formed inside the contact hole and is connected to the cathode electrode. Alternatively, the gate electrode can be a data electrode and the cathode electrode can be a scan electrode. As such, a cathode electrode can be one of a scan electrode and a data electrode, and a gate electrode can be the other one of the scan electrode and the data electorde according to the structure of the EED.
In operation, an electron emission display using the above described configuration concentrates high fields on an acute cathode, that is, an emitter, to emit electrons according to the quantum-mechanical tunnel effect. The electrons emitted from the emitter are accelerated by the voltage applied between the cathode electrode and an anode electrode and are collided with a red, green, and blue (RGB) phosphor layer formed on the anode electrode, thereby emitting one or more lights to display one or more images.
FIG. 1 and FIG. 2 represent a conventional EED 100. FIG. 1 is a partial perspective view of a display panel of the conventional EED 100 and FIG. 2 is a cross-sectional view of a pixel part of the conventional EED100.
As illustrated in FIG. 1 and FIG. 2, the conventional EED 100 includes a rear substrate 1 and a front substrate 2. In addition, the EED 100 includes an emitter 30 (shown in FIG. 2) that function as an electron emission source and electrodes to emit electrons 60 from the emitter. The electrodes include a cathode electrode 10 and a gate electrode 20 that are formed on the rear substrate 1. On the front substrate 2, facing the rear substrate 1, an anode electrode 40 for attracting the electrons 60 emitted from the emitter 30 is formed, and a phosphor layer 50 including RGB phosphors against which the emitted electrons 60 collide and emit light(s) is formed on the anode electrode.
In an EED, brightness and gray scales of images made by the collision of emitted electrons with a phosphor layer and an emission of the phosphors of the phosphor layer are varied according to the input digital video signal. To control the brightness and the gray scales represented by the digital video signal, conventionally Pulse Width Modulation (PWM) and Pulse Amplitude Modulation (PAM) are used.
The PWM is a method for modulating a pulse width of a driving waveform applied to a corresponding electrode according to digital video signals input by a driver to allow emission of a predetermined amount of electrons and control the time of emission, and the PAM is a method for modulating the amplitude of a driving waveform applied to a corresponding electrode by the driver to maintain the time of emission at a predetermined rate and control the amount of momentary electron emission.
In the case of driving a data line using the PWM, conventionally based on an input horizontal synchronous signal together with a video signal, a pulse for representing gray scales is generated. But when the input synchronous signal is unstable and changes, the period or the duration of the active period of the horizontal synchronous signal can be lower than a constant on-time (and/or blank-time) for representing full gray scales and/or 256 gray scales. On this account, the pulse of an upper portion of the lower gray scales cannot be properly represented, therefore full gray scales and brightness can be decreased.